1. Field of the Invention
The present invention relates to an image forming apparatus having a high-voltage power supply.
2. Related Background Art
In FIG. 4, reference numerals 1a to 1d denote photosensitive drums on which toner images are formed on the basis of an electrophotographic process. Reference numeral 2 denotes a transferring belt to which the toner images formed on the photosensitive drums are transferred in the order of 1a to 1d. Reference numerals 3a to 3d denote transferring blades to which predetermined high-voltage outputs are provided using predetermined timings in order to transfer the toner images to the transferring belt 2. Reference numerals 4a to 4d denote transferring high-voltage power supplies that respectively supply predetermined high-voltage outputs to the transferring blades 3a-3d. Reference numeral 5 denotes a cleaner blade that scrapes stains such as toner remaining on the transferring belt 2. Reference numeral 6 denotes a brush provided inside the transferring belt 2 in order to eliminate static electricity from the transferring belt 2 charged by the above described application of the transferring high voltage. Reference numeral 7 denotes a high-voltage power supply that supplies a predetermined static-electricity eliminating high voltage to the brush 6.
In the illustrated configuration, a transferring sheet 8 is supplied from the right of the drawing. The transferring sheet 8 attracted to the transferring belt 2 moves synchronously with rotation of the transferring belt 2 and reaches the photosensitive drum 1a. At this time, a toner image has already been formed on the photosensitive drum 1a and is transferred to the transferring sheet 8 by the transferring blade 3a and a predetermined high-voltage output to the blade 3a supplied from the transferring high-voltage power supply 4a. Subsequently, the transferring sheet 8 is conveyed to the photosensitive drums 1b, 1c, and 1d, where the respective toner images are transferred to the transferring sheet at the respective positions so as to be superimposed on one another. The transferring sheet then passes through a fixer (not shown) provided at the left end of the transferring belt 2, where the toner images superimposed on one another on the transferring sheet are fixed to the transferring sheet 8. On the other hand, the transferring belt 2 continues to rotate after the transferring sheet 8 has been discharged. The cleaner blade 5 scrapes residual toner off. Furthermore, a high-voltage output by the high-voltage power supply 7 supplied to the transferring brush 6 eliminates the charges by the transferring high-voltage output. The transferring belt 2 is weakly negatively charged in order to allow the supplied transferring sheet to be stuck thereto.
FIG. 5 shows an operation of a transferring process on the transferring blade 3a. In the drawing, reference character t denotes the elapse of time counted with reference to the transferring blade 3a. Reference character ta denotes the point of time when the transferring sheet 8 passes through the transferring blade 3. Reference character tb denotes the point of time when the leading end of an image transferred to the transferring sheet 8 reaches the transferring blade. Further, the drawing shows on its axis of ordinates a variation in the voltage of the transferring high-voltage output supplied to the transferring blades. The voltage measured before the transferring sheet 8 reaches the transferring blade 3a is defined as V0. The transferring high-voltage power supply 4a is controlled so that the voltage starts to increase immediately after the transferring sheet 8 has passed through the transferring blade 3a and so that a desired transferring voltage is reached before a toner image on the photosensitive drum 1a reaches the transferring blade 3a. 
The above operation of transferring a toner image is similarly repeated for the photosensitive drums 1b, 1c, and 1d to superimpose toner images of four colors including cyan, magenta, yellow, and black on one another on the transferring sheet 8. Thus, a full color image is formed on the transferring sheet 8.
The voltage V0 in the drawing is weakly negative by the output from the above described high-voltage power supply 7. Accordingly, when the transferring sheet 8 is absent, even if the transferring high-voltage output supplied to the transferring blade 3 is set at 0 V, a current flows from the transferring blades 3a to 3d toward the transferring belt 2. This current serves to charge the photosensitive drum 1 via the transferring belt 2 and form an electrostatic latent image on the photosensitive drum 1. When toner sticks to the electrostatic latent image, it is transferred to a conveyed transferring sheet 8 and a band-like stain image appears on a transferred image to be primarily formed.
To prevent such a stain image, the transferring high-voltage power supplies 4a to 4d are so structured to allow the output of polarity reverse to the inherent transferring high-voltage output, so that the current from the transferring blades 3a to 3d is controlled to 0 xcexcA at the place where the transferring sheets 8 is absent.
The previously described conventional high-voltage power supply apparatus has the following problems:
As described above, the outputs from the transferring high-voltage power supplies 4a to 4d at the place where the transferring sheets 8 is absent must be controlled to 0 xcexcA. However, a control circuit for the transferring high-voltage power supplies itself has a margin of error, so that the current outputs must have a certain tolerance. However, as previously described, the current flowing from the transferring blades 3a to 3d toward the transferring belt 2 (positive direction) causes stain images and is thus intolerable. Thus, a method has been put to practical use which sets the outputs from the transferring high-voltage power supplies 4a to 4d to be weakly negative where the transferring sheet 8 is absent, so as to allow a very small current to flow from the transferring belt 2 toward the transferring blades 3a to 3d (negative direction). However, with this method, a current of minus several xcexcA, which may include a margin of error, may flow. Accordingly, in order to minimize this current, the magnitude of the error must be reduced, thereby making the configuration of the transferring high-voltage power supplies 4a to 4d difficult. On the other hand, an attempt to tolerate a large current in the negative direction causes the photosensitive drum 1 to be charged with a current flowing in the direction opposite to the one previously described. Then, inversely biased toner may be developed, also staining the transferred image.
It is an object of the present invention to provide an image forming apparatus that solves the above described problems.
It is another object of the present invention to provide an image forming apparatus that prevents the adverse effects of a transferring device on image formation while no high voltages are being applied to the transferring device.
It is yet another object of the present invention to provide an image forming apparatus having a transferring device that transfers a developed image resting on an image carrier to a medium, a high-voltage power supply that applies a high voltage to the transferring device, and a disabling device that disables the flow of a current through the transferring device when the above described high-voltage power supply doesn""t apply any high voltage to the above described transferring device.
It is still another object of the present invention to provide an image forming apparatus having a first high-voltage power supply that applies a high voltage to a load, a second high-voltage power supply that applies a high voltage having an opposite polarity compared to the first high-voltage power supply, and a current disabling device that disables the flow of a current from the second high-voltage power supply.
Other objects and features will be apparent from the following specification and drawings.